During extracorporeal blood treatment, for example dialysis, due to a concentration gradient between the blood and the dialysate, a diffusive transport process takes place between the blood and the dialysate.
Said diffusive transport process is substantially important for small-molecular uremia toxins diffusing into the toxin-free dialysate depending on their concentration in the blood and for electrolytes and buffer substances that have either to be removed from the blood (potassium, phosphate) or to be added to the blood (calcium, buffer). The level of concentration of the electrolytes and the buffers in the dialysate determines the rate of the diffusion through the dialysis membrane. The selection of the dialysate composition allows to influence and adapt the diffusion process.
Dialysate is required in large quantity. In the case of dialysate flow of e.g. 500 ml/min, the dialysate quantity for a four to five hours' dialysis treatment amounts to approx. 120 to 150 liters. The dialysate is produced by diluting concentrate with purified osmotic water. The possibility of varying the dialysate composition by specific admixture of individual components permits a dialysis treatment which is tailored to individual needs of the patient.
For use in hemodialysis, dialysates have to be provided which are adapted as properly as possible in the physiological respect, i.e. which have a pH value of about 7.4, contain essential electrolytes and further include a buffer system which is physiological and is suited for adjusting the desired pH value. Since bicarbonate also represents the physiological buffer of blood, generally bicarbonate is used as a buffer system.
In an apparatus for extracorporeal blood treatment, for example a dialysis machine, a so-called dialysate block (DF block) serves as components or parts carrier for, inter alia, conductivity probes and temperature sensors of the dialysate preparation. Usually, principal components of the dialysate preparation are at least one bicarbonate pump and at least on concentrate pump comprising various conductivity measuring cells as well as at least one flow pump. Bicarbonate concentrate admixed via the bicarbonate pump is combined in a mixing chamber and is measured by a conductivity measuring cell. The concentrate or acid concentrate is mixed according to the same principle and is measured by a further conductivity measuring cell.
Temperature sensors are responsible for temperature compensation of the conductivity measurement. The temperature detection of a first temperature sensor is performed after adding cold concentrate (recording of second measured value for the temperature system) and the temperature detection of a second temperature sensor (and of a third temperature sensor) is performed directly ahead of the dialyzer and thus serves for compensating temperature losses.
The conductivity measuring cell or probe is an independently operating monitoring unit. The temperature compensation is performed via a further temperature sensor.
If proportioning is carried out in an apparatus for extracorporeal blood treatment, such as a dialysis machine, with check valves and mixing chambers so as to be capable of using an evaluation of measuring signals of the conductivity probes, permeate and concentrates have to be mixed in a complicated manner in mixing chambers, as the conductivity is measured immediately after admixing the concentrates, so as to obtain an evaluable signal by a previously utilized low-pass filter algorithm.
Moreover, a known solution for measuring the conductivity signal in the apparatus requires a large surface area and/or space and the manufacture of the mixing chambers and of the check valves involves high production and assembly costs.